This invention relates generally to anti-friction bearing type spindles, shafts or axles, and more particularly, to an apparatus to control the preload force on a set of angular contact bearings which are exposed to reversing thrust loads.
When angular contact bearings are used to control the radial and/or axial displacement of the shaft, they are typically used in preloaded pairs (or multiple sets) oriented in a front-to-front, a back-to-back, tandem or any suitable combination of these mounting arrangements.
The pair of bearings are generally preloaded, such that any force exerted on the shaft in either axial direction will instantly encounter substantial resistance by the respective bearing, which is to support the load in that direction, with minimal shaft deflection.
Preload is a parasitic load imposed on the bearings for the dual purpose of controlling shaft deflections from externally applied loads and maintaining proper bearing geometry and frictional forces within the bearing for efficient performance. If the shaft is being exposed to varying speeds and loads, then it is often desired to vary the preload to obtain optimized performance.
U.S. Pat. No. 2,314,622 shows a bearing mount which involves a resilient member which applies a preload to a shaft. There are many other designs which control the preload force which is exerted upon the bearings.
U.S. Pat. No. 4,551,032 shows a spindle which has bearing members attached. A yieldable member, whose flexibility is controlled by the pressure of a fluid which is forced into a cavity in the yieldable member, and thus controls the preload which is applied to the bearings.
U.S. Pat. No. 4,850,719 shows a variable stiffness angular contact bearing wherein the stiffness of the bearing is controlled by piezoelectric wafers which control the preload applied to the bearings.
While all of the above are variable preload devices, an increase in the actuating force which applies the preload may not always result in a similar increase in force being applied to the bearings. The static frictional forces between the bearing housing interface is significant in comparison to the desired variations in preload. This static frictional force is not uniform and as a result cannot be accurately compensated in most variable preload systems. It is difficult to precisely vary the preload directly applied to the bearing, for these reasons the bearings can be overloaded if frictional forces are not adequately accounted for. A precise mechanical preload varying device is costly and requires space.
The varying preload forces of the above patents are all applied directly through the roller elements. Since the roller elements typically cannot take excessive forces, the magnitude of the forces applied by the preload are generally quite small. In all of the hydraulic and pneumatic preload systems, this results in a substantial increase in mechanical compliancy in a thrust direction.
The foregoing illustrates limitations known to exist in present bearing preload control systems. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.
SUMMARY OF THE INVENTION
In one aspect of the present invention, this is accomplished by providing an apparatus including a shaft, with a first and a second angular contact bearing axially disposed along the shaft for supporting the shaft in a first and a second axial direction, respectively. The second axial direction is substantially opposite the first axial direction.
A preload device exerts a preload force onto the angular contact bearings, which place the angular contact bearings into a maximum preload condition. An actuator device, acting in opposition to the preload device, reduces preload from the maximum preload condition to a desired preload condition.
The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing figures.